1. Inoculation of g-IFN-treated human neuroblastoma (NB) cells with the Sabin poliovirus vaccine strain resulted in significantly inhibited viral replication in comparison to replication in undifferentiated, untreated cells. Inhibition of viral replication was one log greater for the vaccine than for the vaccine-revertant strain. This sensitivity to the effects of g-IFN was restricted to the NB cell line. Neither of two other human cell lines, the epithelial tumor HEp-2 and the diploid fetal fibroblast Wi-38 cell lines, distinguished between the two viral strains. This suggests that g-IFN treated NB cells may provide the basis for an in vitro assay for the detection of increased neurovirulence in type 3 poliovirus vaccine strains. 2. It was discovered that a few, g-IFN-treated NB cells survived the cytolytic effects of viral infection. Continuous cultivation of these 'survivors' has resulted in the establishment of persistently infected cell populations which are now ten months old. This work may have relevancy to post-polio syndrome. Although, to date, no trace of live virus has been found, there is immunological evidence indicative of the presence of active viral infection in some patients. A manuscript has been accepted for publication.